The present invention relates to methods of fabricating structural members and, more particularly, to a technique for joining load bearing, thin walled, tubular members so that they possess improved strength and rigidity.
Many products are constructed from tubing members, because a thin walled tube is structurally efficient. Products comprising thin walled tubes have reduced material content and therefore lower weight and cost.
Historically, tubular structures were fabricated by joining tubes to other tubes and end fittings by means of welding. This practice, however, does not provide efficient products because, when the tubular material is welded at its ends, yield strength in the area of the weld is reduced as a result of the local annealing. To increase the diminished strength, the tubular product must then be thickened, thus increasing its material and weight. The increase in material and weight defeats the very purpose of using the thin walled construction in the first place.
Adhesive bonding has been used for certain thin walled tubular applications in order to obtain a weight advantage. However, adhesively bonded joints are not as strong as welded joints or as the material of the tube.
The load bearing joints have been created by a process known as electromagnetic forming (EMF). EMF has been successfully employed to swage electrically conductive tubing onto fittings. The swaging is accomplished by an electromagnetic-induced force. For those applications in which the joint must resist torque, a groove pattern with axes parallel to the centerline of the tube is required. In order to minimize the stress on the joint, the length of joint and fitting may produce a fitting weight which is greater than desired.
A process described in U.S. Pat. No. 3,520,049 to Lysenko.et al, which is referred to as Magnetic Pulse Welding (MPW), is similar to the EMF process. MPW uses magnetic fields induced by high electrical currents to join tubular material to fittings. In the case of MPW, higher forces are generated and the tubular material and fitting are welded together. A torque carrying the MPW joint is shorter and therefore lighter than an equivalent performance EMF joint.
In the MPW process, precise positioning of the structural components is required to produce a consistent weld. A loose fit between the fitting and the tube, as described in U.S. Pat. No. 5,981,921, issued to Yablochnikov on Nov. 9, 1999, requires accurate tooling to support the individual components being welded. YABLOCHNIKOV describes a method for securing components of a vehicular driveshaft. A neck of an end fitting is disposed into the open end of a driveshaft tube. The end fitting is held with respect to the tube so that an annular gap is formed between the neck and the tube. An inductor, disposed about the tube, is energized to generate a magnetic field for collapsing the tube about the neck at a high velocity so that the tube and the end fitting are welded to each other.
The current invention precisely aligns and positions the tube and fitting relative to each other, without external holding fixtures. This precise alignment ensures a high quality weld.
The present invention relates to a method of fabricating structures with load carrying joints by the attachment of tubular components to fittings. A load carrying joint is a joint which carries torsion, bending or tension either individually or in combination. Examples incorporating load bearing joints are: 1) a bicycle frame or wheelchair, 2) the joints in an automobile frame, 3) joints between the end fittings and tubular sections of aircraft torque tubes and push rods, 4) the joint between a golf club shaft and club head, 5) the joint between the end fittings and tubular sections of automotive drive shafts and half shafts, etc.
This invention relates to the fabrication of load bearing joints that are formed from a tubular component and a self-aligning and locating fitting. The resultant joint is capable of handling torque, bending, tension, or a combination of these loads. The method of the invention utilizes a large electrical current to induce a magnetic field that causes the components to be welded to one another, without subjecting the components to the heat associated with conventional welding. Accordingly, the structural joint includes an outer tubular component with an open end. The open end receives an inner, cylindrically shaped member of a fitting. The cylindrically shaped member has a circumferential protuberance that is larger in diameter than the inside diameter of the outer tubular component. This feature acts as a tube stop. The fitting also incorporates an annular locating ring which provides an interference fit with the inside diameter of the outer tubular component. The annular locating ring incorporates a lead chamfer to guide the fitting as it is thrust into the open end of the outer tubular component.
Energizing an inductor positioned around the outer tube at a longitudinal location between the annular locating ring and the tube stop generates a magnetic field that welds the components (i.e., the tube and the fitting) to each other. The corner of the annular locating ring adjacent the tube, closest to the tube stop, is radially shaped. This radial shape reduces local stresses imposed upon the tube during the welding process. This stress relief prevents cracking and improves the fatigue life of the assembly.
It is an object of this invention to provide an improved tubular load bearing joint.
It is another object of the invention to provide a tubular load bearing joint that has improved fatigue life.